Liquid crystal displays (LCDs) of relatively small size have been commercially available for over two decades. Recent improvements have permitted development of larger size displays adapted to use in portable computers, notebook computers and word processors. Full color LCD panels are also used for intermediate-sized, that is 15 cm (6") or smaller, flat screen TVs, projection television systems and camcorder view finders, with many more applications anticipated in the future.
Such display panels may take two forms: passive matrix and active matrix liquid displays (AMLCDs). The passive matrix employs transparent electrodes. These electrodes are patterned in perpendicular, striped arrays on facing glass plates. Red, green and blue color filters on the inner surface of one of the glass plates provide the full color display. The passive matrix display is easier to fabricate than an AMLCD. However, it is more complex to operate, and is limited in performance capabilities.
The active matrix display panel employs transistors or diodes placed on the back cover glass as on-off switches. There is an individual switch for each pixel in a display panel. The electrode film on the front face may be continuous and unpatterned as a consequence. Fabrication of an AMLCD involves three steps: First, the front glass substrate is prepared, including provision of a color filter; second, transistor arrays are formed on the back substrate; third, the two substrates are assembled, and a liquid crystal material inserted within the resulting panel.
Various methods of forming color filters have been proposed. Currently, such filters are produced by separately forming each color pattern on a substrate. This is a repetitive process involving photolithography. Three primary color elements are formed, each typically about 100.times.300 microns in size for the most common application: notebook computers. A black border is also applied around each color element for contrast. The process has been recognized as too expensive to render the product economically attractive for development to its full potential.
K. Mizuno and S. Okazaki, in Japanese Journal Of Applied Physics, Vol. 30, No. 118, November, 1991, pp. 3313-3317, propose producing a color filter by a process wherein ink patterns are successively prepared on a transfer (offset) roll and cured by UV exposure. Each ink pattern is individually transferred to a glass substrate coated with an adhesive layer. It would, of course, be desirable to collect and transfer a complete pattern, and to do so without the need for an adhesive layer. In addition to the items mentioned above, attention is also directed to a publication by W. C. O'Mara entitled "Active Matrix Liquid Crystal Displays Part I: Manufactured Process" appearing at pages 65-70 in the December 1991 issue of SOLID STATE TECHNOLOGY.
It has also been proposed to produce a color filter by photolithography in the form of a film. The pattern may then be inspected, and, if necessary, discarded without printing. If the pattern is accepted, the film is transferred directly to the glass substrate. This proposal is described by K. Ikiaki in a publication entitled "Low Cost Technology for Producing LCD Color Filters Transfer Print Method" In Nikkei MI, Vol:58, pp. 83-87 (90-04). The process still involves photolithography.
U.S. Pat. No. 4,445,432 (Ford et al.) discloses a method and apparatus for forming a multicolor pattern. In this process, thermoplastic decorating inks are printed onto a substrate utilizing a double offset technique. Each color ink is successively printed onto a collector roll to form a fully registered, multicolored pattern on the collector roll. This multicolored pattern is then transferred to the substrate in a single printing step. The process achieves superior registration to that obtained with conventional offset gravure.
U.S. Pat. No. 4,549,928 (Blanding et al.) describes using this double offset technique for printing the phosphors and the black matrix on color TV panels. In this operation, thermoplastic, pressure-sensitive inks, corresponding to the red, green and blue color phosphors and the black matrix, are applied separately to the collector roll to form the desired multicolor pattern. This pattern is then transferred as a complete pattern to the TV tube panel.
The double offset printing technique disclosed in these patents employs pressure-sensitive, hot-melt inks. These inks are printed from heated gravure rolls. The inks cool sufficiently on the offset surfaces to develop the cohesive strength necessary to achieve 100% ink transfer between the offset surfaces and the collector roll, and between the collector roll and the substrate.
The hot-melt inks have several disadvantages for color filter printing. Being pressure-sensitive, hot-melt inks, they are not stable at elevated temperatures as required for color filters. The ingredients typically employed tend to undergo oxidative degradation, or volatilize, at, or prior to, a temperature of 250.degree. C.
It is possible to develop hot-melt inks which can be subsequently cured thermally or by radiation. However, the heated ink procedure is not preferred for precise registration. Slight temperature variations, either in the print surface, or through conduction into the printing apparatus, can result in registration variability.
Consequently, an ideal ink should be capable of ambient temperature printing. It should also achieve 100% transfer without needing to precoat the substrate with an adhesive layer. Moreover, the ink should be able to exhibit stability to at least 250.degree. C.
A basic purpose of our invention is to provide an improved color filter for an LCD panel, in particular an AMLCD panel. Another purpose is to provide an improved method of printing color filters for LCD panels. A further purpose is to provide inks that are particularly adapted to use in printing color filters for LCD panels. Another purpose is to adapt the double offset printing technique to the production of color filters.